1. Technical Field
The present disclosure relates to a magnetic-field sensor, in particular to a sensor that uses anisotropic magnetoresistive (AMR) elements in order to detect a magnetic field that is external to the sensor and acts on the sensor itself.
2. Description of the Related Art
Magnetic-field sensors, in particular AMR sensors based upon anisotropic magnetoresistance, are used in a plurality of applications and systems, for example in compasses, in systems for detection of ferromagnetic materials, in the detection of currents, and in a wide range of other applications, thanks to their capacity for detecting natural magnetic fields (for example, the Earth's magnetic fields) and magnetic fields generated by electrical components (such as electrical or electronic devices and lines traversed by electric current).
The phenomenon of anisotropic magnetoresistivity occurs within particular ferromagnetic materials, which, when subjected to an external magnetic field, undergo a variation of resistivity as a function of the characteristics of the magnetic field applied. Usually, said materials are shaped in thin strips so as to form resistive elements, and the resistive elements thus formed are electrically connected together to form a bridge structure (typically a Wheatstone bridge).
Moreover known to the art is the production of AMR sensors with standard techniques of micromachining of semiconductors, as described, for example, in U.S. Pat. No. 4,847,584. In particular, each magnetoresistive element can be formed by a film of magnetoresistive material, such as for example Permalloy (a ferromagnetic alloy containing iron and nickel), deposited to form a thin strip on a substrate made of semiconductor material, for example, silicon.
When an electric current is made to flow through a magnetoresistive element, the angle θ between the direction of magnetization of said magnetoresistive element and the direction of the flow of the current affects the effective value of resistivity of the magnetoresistive element itself so that, as the value of the angle θ varies, the value of electrical resistance varies (in detail, said variation follows a law of the cos2θ type). For example, a direction of magnetization parallel to the direction of the flow of current results in a maximum value of resistance to the passage of current through the magnetoresistive element, whereas a direction of magnetization orthogonal to the direction of the flow of current results in a minimum value of resistance to the passage of current through the magnetoresistive element.
AMR sensors further include a plurality of straps integrated in the sensors themselves, typically two straps, known as “set/reset strap” and “offset strap”, which are designed to generate, when traversed by a current of an appropriate value, a magnetic field that couples in a direction perpendicular to the direction of detection of the sensors and in the direction of detection of the sensors, respectively; in this regard, see for example U.S. Pat. No. 5,247,278.
The set/reset strap has the function of varying, alternating it, the orientation of magnetization of the magnetoresistive elements in a first pre-defined direction (the so-called “easy axis” or EA). In use, the variation of the sense of magnetization is obtained by applying to the magnetoresistive element, via the set/reset strap, a magnetic field of an appropriate value for a short period of time, such as to force arbitrarily the orientation of the magnetic dipoles of the magnetoresistive element in the first pre-defined direction and with a certain sense (“set” operation), and then by applying to the magnetoresistive element a second magnetic field, similar to the previous one but with opposite sense, so as to force the orientation of the magnetic dipoles of the magnetoresistive element once again in the first pre-defined direction, but with a sense opposite with respect to the previous one (“reset” operation). The set and reset operations have the function of bringing each magnetoresistive element of the AMR sensor in a respective single-domain state before operating the AMR sensor, for example in order to carry out operations of sensing of an external magnetic field. The set and reset operations are used because as only in the single-domain state are the fundamental properties of linearity, sensitivity, and stability of the magnetoresistive elements controlled and repeatable. The aforementioned set and reset operations are known and described in detail for example in the document No. U.S. Pat. No. 5,247,278.
The offset strap is normally used for operations of compensation of the offset present in the AMR sensor (on account of mismatch in the values of the corresponding electrical components), and/or self-test operations, and/or operations of calibration of the AMR sensor. In particular, the value of the electrical quantities at output from the AMR sensor is, in the presence of the offset strap, a function both of the external magnetic field to be detected and of the magnetic field generated as a result of a current circulating in the offset strap. The offset strap is formed by turns of conductive material, for example metal, set on the same substrate as that on which the magnetoresistive elements of the sensor and the set/reset strap are provided (in different metal layers), and is electrically insulated from, and set in the proximity of, said magnetoresistive elements. The magnetic field generated by the offset strap generates a magnetic field component that tends to shift the orientation of the magnetic dipoles of each magnetoresistive element in a second pre-defined direction (the so-called “hard axis” or HA), orthogonal to the first pre-defined direction, which is the direction of sensitivity of the sensor.
FIG. 1 shows, in top plan view, a layout provided by way of example of an integrated circuit for a magnetic-field sensor 1 of a known type, comprising a plurality of magnetoresistive elements, connected to one another so as to form a Wheatstone bridge, for example as described in U.S. Pat. Nos. 5,247,278 and 5,952,825, and obtained, for example, as described in U.S. Pat. No. 4,847,584.
More in particular, each magnetoresistive element has a structure of the barber-pole type. The barber-pole structure for magnetoresistive elements is known, for example, from U.S. Pat. No. 6,850,057. In this case, each magnetoresistive element is in ohmic contact with a plurality of biasing elements with high electrical conductivity (for example, ones made of aluminium, copper, silver, or gold). The biasing elements are arranged inclined by a certain angle α (typically, α=45°) with respect to the axis of spontaneous magnetization of the magnetoresistive element.
The magnetic-field sensor 1 is formed on a semiconductor substrate 2 by means of micromachining processes of a known type. Four magnetoresistive elements 4, 6, 8, and 10, in the form of strips of ferromagnetic material (for example, deposited thin film comprising an Ni/Fe alloy), in a barber-pole configuration, are arranged to form a Wheatstone bridge. With reference to FIG. 1, the magnetoresistive elements 4, 6, 8, 10 are interconnected to one another and connected to pads 21, 22, 23, 24, and 25. The pad 21 is connected to the magnetoresistive element 4 by means of a conductive path 11, and the magnetoresistive element 4 is connected to the magnetoresistive element 6 by means of a conductive portion 17. The conductive portion 17 is electrically connected to the pad 22 by means of a respective conductive path 12. The magnetoresistive element 6 is then connected to the magnetoresistive element 10 by means of a conductive portion 18, and the conductive portion 18 is electrically connected to the pad 23 by means of a respective conductive path 13. The magnetoresistive element 10 is interconnected to the magnetoresistive element 8 by means of a conductive portion 16, and the conductive portion 16 is electrically connected to the pad 24 by means of a respective conductive path 14. The pad 25 is connected to the magnetoresistive element 8 by means of a conductive portion 15.
A resistive Wheatstone-bridge structure is thus formed, which provides a magnetic-field sensor 1 sensitive to components of magnetic field having a direction perpendicular to the strips of ferromagnetic material that form the magnetoresistive elements 4, 6, 8, 10. The pad 21 is connected to the pad 25, to form a common pad so as to connect the magnetoresistive element 4 and the magnetoresistive element 8 electrically together (in FIG. 1 said pads 21, 25 are shown separated for reasons of optimization of the layout).
In use, an input voltage Vin is applied between the pad 22 and the pad 24. Reading of the output voltage Vout is made between the pad 21 (common to the pad 25) and the pad 23.
With reference to FIG. 1, the magnetic-field sensor 1 further comprises a first strip of electrically conductive material extending on the substrate 2 and insulated from the latter by means of a layer of dielectric material (not shown in detail in the figure). Said first strip of electrically conductive material forms a first winding 19, of a planar type, which extends in a plane parallel to the plane in which the magnetoresistive elements 4, 6, 8, 10 lie, electrically insulated from the magnetoresistive elements 4, 6, 8, 10.
The magnetic-field sensor 1 further comprises a second strip of electrically conductive material, extending on the substrate 2 and insulated from the latter and from the first winding 19 by means of a layer of dielectric material (not shown in detail in the figure). Said second strip of electrically conductive material forms a second winding 20, of a planar type, which extends between a terminal 20a and a terminal 20b in a plane parallel to the plane in which the magnetoresistive elements 4, 6, 8, 10 and the first winding 19 lie, and is electrically insulated from the magnetoresistive elements 4, 6, 8, 10 and from the first winding 19.
The first winding 19 is used when it is desired to generate a magnetic field of known intensity interacting with the magnetic-field sensor 1, for example with purposes of self-testing of the sensor, biasing, calibration, and/or compensation of possible offsets due to the presence of undesirable external magnetic fields. In the latter case, the effect of the magnetic field generated by the first winding 19 on the output signal Vout of the magnetic-field sensor 1 is that of balancing the output signal due exclusively to the undesirable external field in order to generate a zero output signal.
In use, when the first winding 19 is traversed by electric current, a magnetic field is generated, the lines of force of which have a direction along the axis of sensitivity of the magnetoresistive elements 4, 6, 8, 10.
On account of the variability of the process of fabrication of the magnetoresistive elements 4, 6, 8, 10, said magnetoresistive elements 4, 6, 8, 10 can have structural characteristics that differ from one another. Once the bridge is supplied, these different characteristics, which can be equated to differences of resistance on the four branches of the bridge, generate an offset voltage Voff between the output terminals of the bridge. This voltage Voff does not have any relationship with the magnetic field that is to be detected but adds, in an undesirable way, to the signal generated by the transduction of the magnetic field that is to be detected in voltage. The offset voltage Voff can be of the order of the output voltage expected from the magnetic fields that are to be detected, and sometimes even greater. The offset signal Voff can be isolated and removed by appropriately using the second winding 20, during reading of the output signal Vout. In greater detail, in use, current pulses of a high value are made to flow in the second winding 20, in opposite directions with respect to one another (by appropriately biasing the terminals 20a and 20b of the second winding) so as to generate respective magnetic fields defined by respective lines of field having a sense opposite to one another. Said magnetic fields have an intensity such as to re-orient the magnetic dipoles of the magnetoresistive elements 4, 6, 8, 10 according to the lines of field generated, in particular with orientation defined by the orientation of the lines of the magnetic field generated. The effect of this orientation of the magnetic dipoles is basically to change the sign to the sensitivity of the sensor. For what has been said, instead, the offset signal Voff, which has purely electrical origins, is not affected by this change of orientation of the magnetic dipoles.
Following upon a first current pulse (referred to as “set pulse”) through the second winding 20, a first magnetic field BS1 is generated such as to orient the magnetic dipoles of the magnetoresistive elements 4, 6, 8, 10 according to a first sense. A first reading of the output signal Vout of the sensor is then made.
Following upon a second current pulse (referred to as “reset pulse”) through the second winding 20, a second magnetic field BS2 is generated (of intensity, for example, equal to that of the first magnetic field BO such as to orient the magnetic dipoles of the magnetoresistive elements 4, 6, 8, 10 in a second sense. A second reading of the output signal of the sensor is made. If the first and second readings are available and the difference is made between the values obtained from said readings, the offset signal Voff is erased, and it is thus possible to discriminate just the component of magnetic origin of the signal.
In order for the AMR sensor of FIG. 1 to be operated as described, it is coupled with an appropriate circuitry for power supply and generation and acquisition of signals (in order to supply the set/reset and offset straps and to acquire and process the output voltage signal Vout). For this purpose, electrical connections are provided that are designed to carry supply signals (e.g., input voltage Vin) to the pads 22 and 24, and acquire output signals (e.g., output voltage Vout) from the pads 21, 23 and 25.
Generally, the circuitry designed to generate the supply signal Vin and the output signal Vout is implemented in integrated form (for example, as ASIC) on a chip different from the chip on which the AMR sensor is formed. The connections between the chip carrying the circuitry for power supply and generation and acquisition of signals and the terminals of the AMR sensor are consequently formed by means of wire bonding or else using flip-chip techniques, through bumps. As may be clearly appreciated from FIG. 1, given a sensitive part having all things considered contained dimensions, the overall area of the magnetic sensor may be considerably larger on account of the presence of the straps, in particular the set/reset strap, and on account of the presence of the pads for communication between the sensor and the control/acquisition electronics. In addition, the process (which is relatively simple) of deposition of the ferromagnetic material, is rendered burdensome by numerous successive process steps to provide various metal levels, considerably increasing the manufacturing costs in terms of number of masks and process cost (machinery, process steps, manufacturing times). In addition, given the presence of current peaks of a high value, there may arise problems due to the contacts, in particular on account of generation of parasitic signals (parasitic currents and voltages, parasitic capacitances, etc.), and/or problems of reliability of the contacts provided by means of bonding and/or using bumps, which can jeopardize proper operation of the sensor.
What has been said must consequently be taken into account in the design of the AMR sensor and in the analysis of the output signal Vout acquired from the latter by the acquisition circuitry, which causes an increase of the hardware and/or software complexity of the AMR sensor and/or of the circuitry for power supply and generation and acquisition of signals.